New, transforming, murine, type C viruses were characterized with respect to their genome structure and their ability to transform cells in vitro and in vivo: (1) A lung carcinoma-associated virus (LCAV, CI-1-4) was molecularly cloned and shown to have a mink cell focus-forming (MCF)-type structure. Transformed mink lung cells harboring this virus were sensitized to epidermal growth factor (EGF), suggesting that the mechanism of transformation by LCAV may involve sensitization of alveologenic lung cells to EGF. Two of the molecularly cloned viruses, CI-3 and CI-4 were also tested in vivo and found to be oncogenic. The nucleic acid sequence of CI-3 virus showed that the entire envelope gp70 was derived from an endogenous dual-tropic env gene. Comparison of the CI-3 viral gp70 and LTR sequences with those of class I and class II MCF MuLV established its unique structure. (2) A new oncogene, raf, from the murine sarcoma virus, 3611-MSV, was molecularly cloned, sequenced, and used for the isolation of its human cellular homologs, which were mapped on human chromosomes 3 and 4. The translational products of this virus were P90 and P75 polyproteins which contained viral p15 and p12 structural proteins fused to a tumor gene product lacking tyrosine-specific phosphokinase activity. The two polyproteins differ only by the fact that P90 is in a glycosylated form, whereas P75 is myristilated. Synthetic peptide sera, as well as a polyvalent, anti-v-raf, protein serum, identify a 60K, cytosolic, c-raf-1 protein in human cells. This size agrees well with the size predicted from the nucleotide sequence of the human c-raf-1 proto-oncogene. The raf protein was purified for amino acid sequence determination from mouse fibroblast cells in which the c-raf proto-oncogene was activated by an LTR insertion and subsequently amplified. Transformation of cells by the raf oncogene is enhanced in vivo and in vitro in the presence of a second oncogene, v-myc. A combination of both oncogenes was discovered in the genome of the avian carcinoma virus, MH2, suggesting a role for this specific pair of genes in the development of natural carcinomas.